Possibly abut piece of brass (4)
I believe the answer is:
tuba
'piece of brass' is the definition.
'tuba' can be an answer for 'brass' (tuba is a kind of brass). I am not sure about the 'piece of' bit.
'possibly abut' is the wordplay.
'possibly' indicates anagramming the letters.
'abut' with letters rearranged gives 'TUBA'.
Can you help me to learn more?
(Other definitions for tuba that I've seen before include "Large low-pitched brass musical instrument" , "Brass band instrument" , "It's not top brass" , "One's blown" , "Stop" .)